Synchronization

ABSTRACT

A mechanism for achieving frame synchronization in the frequency domain. In order to synchronize a receiver with a transmitter, on signal acquisition, the interval in which orthogonality exists is determined. Once this has been achieved, an argument function is calculated from the received frame. This argument function can then be used to improve the synchronization. This system is particularly suitable for use in ADSL and VDSL modems which can be used to give broad band access over copper networks. It is also relevant to broad band transmission in mobile and semi-mobile systems for transmission over radio channels.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an OFDM transmission system, an OFDMreceiver, OFDM modems including ADSL modems and VDSL modems, and methodsof synchronising an OFDM receiver with an incoming multi-carrier signal,in particular, the present invention relates to frame synchronisationfor an OFDM system using frequency domain data.

2. Background of the Invention

In this specification the term OFDM (Orthogonal Frequency DivisionMultiplex) type is intended to include DMT (Discrete Multi-Tone).

The demand for provision of multi-media and other broad bandwidthservices over telecommunications networks has created a need to transmithigh bit rate traffic over copper pairs. This requirement has led to thedevelopment of a number of different transmission schemes, such as, ADSL(Asynchronous Digital Subscriber Line) and VDSL (Very high bit-rateDigital Subscriber Lines). One of the more likely modulation systems forall these transmission schemes is a line code known as DMT (discretemulti-tone), which bears a strong resemblance to orthogonal frequencydivision multiplex, and is a spread spectrum transmission technique.

In discrete multi-tone transmission, the available bandwidth is dividedinto a plurality of sub-channels each with a small bandwidth, 4 kHzperhaps. Traffic is allocated to the different sub-channels independence on noise power and transmission loss in each sub-channel.Each channel carries multi-level pulses capable of representing up to 11data bits. Poor quality channels carry fewer bits, or may be completelyshut down.

Because inter pair interference in copper pair cables is higher wheredata is transmitted in both directions, i.e. symmetric duplex, a numberof transmission schemes have proposed the use of asymmetric schemes inwhich high data rates are transmitted in one direction only. Suchschemes meet many of the demands for high bandwidth services, such as,video-on-demand.

VDSL technology resembles ADSL to a large degree, although ADSL mustcater for much larger dynamic ranges and is considerably more complex asa result. VDSL is lower in cost and lower in power, and premises VDSLunits need to implement a physical layer media access control formultiplexing upstream data.

Four line codes have been proposed for VDSL:

CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passiveNT configurations, CAP would use QPSK upstream and a type of TDMA formultiplexing (although CAP does not preclude an FDM approach to upstreammultiplexing);

DMT; Discrete Multi-Tone, a multi-carrier system using Discrete FourierTransforms to create and demodulate individual carriers, for passive NTconfigurations; DMT would use FDM for upstream multiplexing (althoughDMT does not preclude a TDMA multiplexing strategy);

DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using WaveletTransforms to create and demodulate individual carriers, DWMT also usesFDM for upstream multiplexing, but also allows TDMA; and

SLC; Simple Line Code, a version of four-level baseband signalling thatfilters the base band and restores it at the receiver, for passive NTconfigurations; SLC would most likely use TDMA for upstreammultiplexing, although FDM is possible.

Early versions of VDSL will use frequency division multiplexing toseparate downstream from upstream channels and both of them from POTSand ISDN. Echo cancellation may be required for later generation systemsfeaturing symmetric data rates. A rather substantial distance, infrequency, will be maintained between the lowest data channel and POTSto enable very simple and cost effective POTS splitters. Normal practicewould locate the downstream channel above the upstream channel. However,the DAVIC specification reverses this order to enable premisesdistribution of VDSL signals over coaxial cable systems.

In a multi-carrier system, such as a DMT system, a receiver must be ableto recover a sampling clock that is very accurately synchronized to atransmitter sampling clock. A known method, for achievingsynchronization, uses a reserved carrier, the pilot carrier, which istransmitted with a fixed phase. The receiver sampling clock is thenphase locked to the pilot carrier. Frame timing must also be recovered.In existing systems this has been achieved by using a correlationtechnique operating in the time domain.

With OFDM systems the frequency domain data is the Fourier transform ofthe received time domain OFDM frames. The time domain frames must besampled, at the receiver, in synchronism with the transmitter, so thateach received frame contains data from only a single transmitted frame.It is vital for this synchronism to be maintained in order to maintainthe orthogonality of the frames.

A typical multi-carrier system, of the OFDM type, which uses a cyclicprefix, permits orthogonality to be maintained when there is a smalldeviation from exact frame synchronisation. Because the signallinginterval includes both an entire frame and the cyclic prefix, which is arepetition of part of the frame, a frame sampled within the signallinginterval will contain data from only one frame. Since the signallinginterval is greater than the frame period, this gives some leeway inframe alignment.

The present invention provides a mechanism for achieving framesynchronisation, in the frequency domain, by utilising this fact. Thefirst step in synchronising a receiver with a transmitter, on signalacquisition, is to determine the interval in which orthogonality exists.Once this has been achieved an argument function is calculated from thereceived frame. This argument function can then be used to improve thesynchronisation.

Known techniques for achieving frame synchronisation do not operateentirely in the frequency domain, as is the case for the presentinvention. Use of the present invention permits implementation of OFDMreceivers with a considerable saving in complexity, compared with priorart receivers, because the arithmetic operations required can beperformed at low resolution.

SUMMARY OF THE INVENTION

The present invention is particularly suitable for use in ADSL and VDSLmodems which can be used to give broadband access over copper networks.The invention is also of relevance to broadband transmission in mobileand semi-mobile systems for transmission over the radio channels.

According to a first aspect of the present invention, there is provideda receiver, for use in an OFDM type transmission system, in which datais transmitted in frames, each frame having a cyclic prefix which is arepetition of part of said frame, characterised in that said receiver issynchronised with transmitted frames by operating on frequency domaininput data.

Search means may be provided for carrying out a search for a time domaininterval of one frame length which falls within a signalling intervalthat includes said frame and said cyclic prefix.

Said search means may include a counter means for generating a framestart pulse, said counter means' state being modified in steps whichshift said frame start pulse by an amount equal to said cyclic prefixlength.

Said counter means may be clocked by a sampling clock.

Said counter means may be clocked to shift the timing of said framestart pulse until an objective function has a minimum value.

Said objective function may be derived from said frequency domain inputdata.

Said objective function may be an estimate of a disturbance in anargument function.

Said objective function, J, for a kth frame may be calculated from:$J_{k} = {\sum\limits_{n}{{\angle( {\frac{X_{n,k}}{T_{n,k}} - {\angle ( \frac{X_{{n - 1},k}}{T_{{n - k},k}} )}} }}}$

where X is a frequency domain vector, and T refers to training framedata and said summation is over all active carriers n.

Said search means may continue to search, until a first sample of aframe is identified, by using an estimated slope of an argument functionof a frequency domain vector, X.

Said counter means' state may be modified incrementally until said slopeis close to zero.

According to a second aspect of the present invention, there is providedan OFDM type transmission system in which data is transmitted in frames,each frame having a cyclic prefix which is a repetition of part of saidframe, characterised in that said system includes a receiver as definedin any preceding paragraph.

According to a third aspect of the present invention, there is provided,in an OFDM type system in which data is transmitted in frames, eachframe having a cyclic prefix which is a repetition of part of saidframe, a method of synchronising a receiver with a transmitter so thatreceived frames are aligned with transmitted frames, characterised byoperating, in said receiver, on frequency domain input data.

Said receiver may search for a time domain interval of one frame lengththat falls within a signalling interval which includes said frame andsaid cyclic prefix.

Said searching step may be performed by incrementally modifying acounter's state, said counter generating a frame start pulse which, foreach increment, is shifted by an amount equal to said cyclic prefixlength.

Said counter may be clocked with a sampling clock.

Said counter may be docked to shift said frame start pulse's timinguntil an objective function has a minimum value.

Said objective function may be derived from said frequency domain inputdata.

Said objective function may be estimated from a disturbance in anargument function.

Said objective function, J, for a kth frame may be calculated from:$J_{k} = {\sum\limits_{n}{{\angle( {\frac{X_{n,k}}{T_{n,k}} - {\angle ( \frac{X_{{n - 1},k}}{T_{{n - k},k}} )}} }}}$

where X is a frequency domain vector, and T refers to training framedata and said summation is over all active carriers n.

Said search may be continued, to identify a first sample of a frame, byusing an estimated slope of an argument function of a frequency domainvector, X.

Said counter may be modified, incrementally, until said slope is closeto zero.

According to a fourth aspect of the invention there is provided an ADSLmodem characterised in that said modem has a receiver as defined above,or operates a method of synchronisation as defined above.

According to a fifth aspect of the present invention, there is provideda VDSL modem characterised in that said modem has a receiver as definedabove, or operates a method of synchronisation as defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 illustrates, in functional form, an equaliser and samplingcontrol unit in which the present invention can be implemented.

FIG. 2 illustrates the time domain data format of an OFDM signal usedwith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The synchronisation process of the present invention is illustrated in afunctional form in FIG. 1. Incoming frequency domain data is passed viaan equaliser to a detector/quantizer and thence to a symbol decoder. Theoperation of the remaining blocks shown in FIG. 1, namely theequalisation parameter updating algorithm, the sampling clock controlalgorithm and the frame timing algorithm are explained in the followingdescription. It is, however, worth noting at this point that:

the equalisation parameter updating algorithm takes inputs from the rawfrequency domain input data, X, the output of the equaliser, U, and theoutput of the detector/quantizer, Y;

the sampling clock control algorithm receives an input from theequalisation parameter updating algorithm, as does the equaliser; and

the frame timing algorithm accepts an input from the raw frequencydomain input data.

The frequency-domain data comprises the received time-domain OFOM framesafter Fourier transformation. The time-domain frames must be sampled insynchronism with the transmitter so that each received frame containsdata from only one transmitted frame. This is important in order tomaintain the orthogonality of the frames.

FIG. 2 shows the time-domain format for the transmission of OFDM framesused with the present invention.

The signalling interval contains a cyclic prefix and a frame. The cyclicprefix is a copy of the last part of the frame. This means that a framesampled anywhere inside the signalling interval will contain data fromone transmitted frame only. A deviation from the exact frame timingwill, therefore, lead to a cyclic permutation of the frame. Theorthogonality will, however, be maintained.

The frame timing deviation can be detected as a linear slope of anargument function—see below for further details, proportional to thedeviation. This will, however, only be valid for as long as theorthogonality is maintained. The cyclically permuted interval (thesignalling interval) must first be found initially.

Frequency-domain synchronisation is thus based on an argument functionof the received frames (X), compensated for the modulation. The argumentfunction of a frequency domain frame is the vector of arguments of theindividual complex elements (carriers).

The argument of an individual carrier, in this case, is the sum of themodulation argument, the channel influence and the sample timingdeviation. The modulation argument is eliminated by subtracting theknown argument value. This is done using known transmitted data (atraining frame T). The channel influence is neglected in this case.

The argument of a complex number is the inverse tangent of the imaginarypart divided by the real part. A problem involved in this calculation isthat the inverse tangent function is periodic with a period 2π radians.In this application, it is necessary to handle arguments larger than nradians, which is the range of the inverse tangent function. It isusually possible to use some other information about the argument tounwrap this periodic function so that it covers a larger range. A usefulassumption is that the difference in argument between adjacent carriersis smaller than π radians. It is then possible to compensate for eachdiscontinuity, caused by the inverse tangent function periodicity, andthus unwrap the argument function.

In order to use the argument function for sample timing control, it isnecessary that the sampling of the received frame always starts insidethe cyclically permuted interval (the signalling interval). If thiscondition is not fulfilled, the argument function will be severelydisturbed and will not be valid for sample timing control.

The present invention employs a new technique for finding the correctinterval for frame sampling and uses an estimate of the magnitude of thedisturbance of the argument function as an objective function.

A frame sampled inside the correct interval shows a minimum value forthis magnitude. The frame start time is stepped through a signallinginterval length and the objective function is estimated for each step.The step size is equal to the cyclic prefix size. The frame start timethat gives the minimum value of the objective function is then selected.

Any measure that represents the magnitude of variations in the argumentfunction can be used as an objective function in this technique. Anexample of such a measure is given by equation (1), below.

The sum of the magnitudes of the argument differences of adjacentcarriers is used as the objective function, J, for selecting the correctframe interval. The divisions by the training frame, T, components,shown in the equation, are not actually performed, since only thearguments of the quotients are calculated (modulation compensation). Anunwrapping function is used to take care of the occasions when argumentdifferences of adjacent carriers are taken across the discontinuity ofthe inverse tangent function. $\begin{matrix}{J_{k} = {\sum\limits_{n}{{\angle( {\frac{X_{n,k}}{T_{n,k}} - {\angle ( \frac{X_{{n - 1},k}}{T_{{n - k},k}} )}} }}}} & (1)\end{matrix}$

The range of n in equation (1) depends on which carriers are active. Incases when the band of active carriers is divided into several parts,separated by empty bands, the objective function J is calculated,according to equation (1), with the inactive carriers omitted. It isimportant that both operands of the difference expression always relateto active carriers.

A minimum value of J means that the frame has been sampled inside thecyclically permuted interval (signalling interval).

The initialization of the frame synchronization is performed in thesteps set out below. Initially, a training frame, modulated with knowndata, is transmitted repeatedly. Then the following synchronizationsteps are implemented:

10. A search is performed for a time-domain interval of one framelength, located inside one cyclically permuted interval (signallinginterval). The frame start pulse is generated by a counter, clocked bythe sampling clock, and has a period equal to the signalling interval.During the search procedure, the counter state is modified in stepsequal to the cyclic prefix length until the correct interval is found,as indicated by the objective function. The frequency domain vector, X,is used as the input to the objective function estimation.

11. The search is continued for the first sample of the frame by usingthe estimated slope of the unwrapped frame argument function of thevector X. The state of the frame start position counter is modified in abinary search fashion (successive approximation), until the slope isclose to zero. The sign of the argument slope is used to determine thedirection of each change. The slope of the argument function isestimated using a standard method.

The unique novelty in the technique of the present invention is the useof the argument function's properties for detecting whether, or not, thesampled frame is orthogonal. This indicates whether, or not, the wholeframe has been sampled correctly inside one signalling interval.

The present invention permits a low complexity implementation, since thearithmetic operations involved can be performed using low resolution.Especially the inverse tangent operation can be greatly simplified.

What is claimed is:
 1. A receiver, for use in an OFDM transmissionsystem, in which data is transmitted in frames, each frame having acyclic prefix which is a repetition of part of said frame, said receiverbeing synchronised with transmitted frames by operating on frequencydomain input data, said receiver comprising search means for carryingout a search for a time domain interval of one frame length which fallswithin a signalling interval that includes said frame and said cyclicprefix, said search means including a counter means for generating aframe start pulse, a state of said counter means being modified in stepswhich shift said frame start pulse by an amount equal to said cyclicprefix length.
 2. A receiver, as claimed in claim 1, wherein saidcounter means is clocked by a sampling clock.
 3. A receiver, as claimedin claim 1, wherein said counter means is clocked to shift the timing ofsaid frame start pulse until an objective function has a minimum value.4. A receiver, as claimed in claim 3, wherein said objective function isderived from said frequency domain input data.
 5. A receiver, as claimedin claim 4, wherein said objective function is an estimate of adisturbance in an argument function.
 6. A receiver, as claimed in claim5, wherein said objective function, J, for a kth frame is calculatedfrom:$J_{K} = {\sum\limits_{n}{{\angle( {\frac{X_{n,k}}{T_{n,k}} - {\angle ( \frac{X_{{n - 1},k}}{T_{{n - k},k}} )}} }}}$

where X is a frequency domain vector, and T refers to training framedata and said summation is over all active carriers n.
 7. A receiver, asclaimed in claim 4, wherein said search means continues to search, untila first sample of a frame is identified, by using an estimated slope ofan argument function of a frequency domain vector, X.
 8. A receiver, asclaimed in claim 7, wherein a state of said counter means is modifiedincrementally until said slope is close to zero.
 9. An OFDM transmissionsystem in which data is transmitted in frames, each frame having acyclic prefix which is a repetition of part of said frame, said systemcomprising a receiver being synchronised with transmitted frames byoperating on frequency domain input data, said receiver comprisingsearch means for carrying out a search for a time domain interval of oneframe length which falls within a signalling interval that includes saidframe and said cyclic prefix, said search means including a countermeans for generating a frame start pulse, a state of said counter meansbeing modified in steps which shift said frame start pulse by an amountequal to said cyclic prefix length.
 10. A method of synchronising areceiver with a transmitter in an OFDM system in which data istransmitted in frames, each frame having a cyclic prefix which is arepetition of part of said frame, so that received frames are alignedwith transmitted frames, wherein the receiver operates on frequencydomain input data, the method comprising said receiver searching for atime domain interval of one frame length that falls within a signallinginterval which includes said frame and said cyclic prefix, saidsearching being performed by incrementally modifying a counter's state,said counter generating a frame start pulse which, for each increment,is shifted by an amount equal to said cyclic prefix length.
 11. Amethod, as claimed in claim 10, further comprising clocking said counterwith a sampling clock.
 12. A method, as claimed in claim 10, furthercomprising clocking said counter to shift said frame start pulse'stiming until an objective function has a minimum value.
 13. A method, asclaimed in claim 12, further comprising deriving said objective functionfrom said frequency domain input data.
 14. A method, as claimed in claim13, further comprising estimating said objective function from adisturbance in an argument function.
 15. A method, as claimed in claim14, further comprising calculating said objective function, J, for a kthframe from:$J_{K} = {\sum\limits_{n}{{\angle( {\frac{X_{n,k}}{T_{n,k}} - {\angle ( \frac{X_{{n - 1},k}}{T_{{n - k},k}} )}} }}}$

where X is a frequency domain vector, and T refers to training framedata and said summation is over all active carriers n.
 16. A method, asclaimed in claim 13, further comprising continuing said search, toidentify a first sample of a frame, by using an estimated slope of anargument function of a frequency domain vector, X.
 17. A method, asclaimed in claim 16, further comprising modifying said counter,incrementally, until said slope is close to zero.
 18. An ADSL modemcomprising a receiver as claimed in claim
 1. 19. A VDSL modem comprisinga receiver as claimed in claim
 1. 20. A receiver, for use in an OFDMtransmission system in which data is transmitted in frames, each framehaving a cyclic prefix which is a repetition of part of the frame, thereceiver being synchronised with transmitted frames by operating onfrequency domain input data, the receiver comprising a search unit tofind a correct frame sampling interval by finding a minimum value of anobjective function derived from the frequency domain input data, and toselect a frame start time associated with the minimum value.
 21. Areceiver according to claim 20, wherein the search unit searches for atime domain interval of one frame length which falls within a signallinginterval that includes the frame and the cyclic prefix.
 22. A receiveraccording to claim 21, wherein the search unit includes a counter forgenerating a frame start pulse, a state of the counter being modified insteps which shift the frame start pulse by an amount equal to the cyclicprefix length.
 23. A receiver according to claim 20 wherein said counteris clocked by a sampling clock.
 24. A receiver according to claim 20wherein said counter is clocked to shift the timing of the frame startpulse until the objective function has a minimum value.
 25. A receiveraccording to claim 24 wherein the objective function is an estimate of adisturbance in an argument function.
 26. A receiver according to claim25 wherein the objective function, J, for a kth frame is calculatedfrom:$J_{K} = {\sum\limits_{n}{{\angle( {\frac{X_{n,k}}{T_{n,k}} - {\angle ( \frac{X_{{n - 1},k}}{T_{{n - k},k}} )}} }}}$

where X is a frequency domain vector, and T refers to training framedata and the summation is over all active carriers n.
 27. A receiveraccording to claim 24 wherein the search unit continues to search, untila first sample of a frame is identified, by using an estimated slope ofan argument function of a frequency domain vector, X.
 28. A receiveraccording to claim 27 wherein a state of the counter is modifiedincrementally until said slope is close to zero.
 29. A method ofsynchronising a receiver with a transmitter in an OFDM system in whichdata is transmitted in frames, each frame having a cyclic prefix whichis a repetition of part of the frame, so that received frames arealigned with transmitted frames, wherein the receiver operates onfrequency domain input data, the method comprising.
 30. A methodaccording to claim 29 wherein searching comprises searching for a timedomain interval of one frame length that falls within a signallinginterval which includes the frame and the cyclic prefix.
 31. A methodaccording to claim 30 wherein searching further comprises incrementallymodifying a counter's state, said counter generating a frame start pulsewhich, for each increment, is shifted by an amount equal to said cyclicprefix length.
 32. A method according to claim 31 further comprisingclocking the counter with a sampling clock.
 33. A method according toclaim 31 further comprising clocking the counter to shift the framestart pulse's timing until an objective function has a minimum value.34. A method according to claim 33 further comprising estimating theobjective function from a disturbance in an argument function.
 35. Amethod according to claim 34 further comprising calculating theobjective function, J, for a kth frame from:$J_{K} = {\sum\limits_{n}{{\angle( {\frac{X_{n,k}}{T_{n,k}} - {\angle ( \frac{X_{{n - 1},k}}{T_{{n - k},k}} )}} }}}$

where X is a frequency domain vector, and T refers to training framedata and the summation is over all active carriers n.
 36. A methodaccording to claim 33 further comprising continuing the search, toidentify a first sample of a frame, by using an estimated slope of anargument function of a frequency domain vector, X.
 37. A methodaccording to claim 36 further comprising modifying the counter,incrementally, until the slope is close to zero.